Load Variation effect on Maximum Power Point Tracker (MPPT) for Solar Photovoltaic (PV) Energy Conversion System

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1 Load Variation effect on Maximum Power Point Tracker (MPPT) for Solar Photovoltaic (PV) Energy Conversion System Ahteshamul Haque Department of Electrical Engineering, Jamia Millia Islamia, New Delhi Abstract Solar energy has got tremendous attention from the researchers among all renewable energy sources in the last few decades. It is available in abundant and free of cost. Solar Photovoltaic (PV) is used to convert the solar energy into unregulated electrical energy. Maximum power point tracker (MPPT) is an algorithm used to extract maximum power from solar PV. Power Electronics converters are used to regulate the power generated from Solar PV. The behaviour of these converters along with MPPT needs to be analysed with load variations. The objective of this paper to design MPPT considering the load factor. A modified P&O MPPT technique is used for this study. The simulation study is done using PSIM simulation software and a prototype is made to validate the results. Keywords Solar Energy, MPPT, Buck converter, Perturb and observe, Hill Climbing, PSIM I. INTRODUCTION Energy has become the basic need of human being. The demand of Electrical energy is increasing exponentially [1]. This increase in demand leads to concerns of global energy crisis as the reservoirs of fossils fuels are limited. The other concern is of environmental threats due the emission of greenhouse gases from fossil fuels i.e. from thermal power plants etc. Among other available sources of energy like nuclear possess serious safety concerns for the human being. This concern leads the researchers to look for alternate sources of electrical energy. Renewable energy sources (RES) has got tremendous attention as an alternate source of electrical energy. RES consist of Solar, Wind, and Tidal etc. Among all renewable energy sources solar energy is considered as the most acceptable source of energy as it is available in abundant, available free of cost and have very less safety concerns [2]. Solar Photovoltaic (PV) is used to convert the solar energy into electrical energy. Solar PV has a nonlinear characteristic as its output varies with solar irradiation and with ambient temperature [3]. Also the other constraint is the efficiency of solar PV is very low. It is essential to extract maximum power from solar PV under normal and varying solar irradiation and ambient temperature condition. To achieve this task an algorithm known as Maximum Power Point tracking (MPPT) is developed [4]. Various types of MPPT algorithms are proposed mainly are fixed duty cycle, constant voltage, short circuit current, Perturb & Observe (P&O), Incremental Conductance (IC) methods etc. Many researchers have designed and evaluated these various types of MPPT techniques [5-9]. The modified power electronics converters are also proposed to improve the performance [10]. Even the MPPT control is modified to adaptive nature for better performance [12-13]. Among all these evaluations the main focus is on the variation of solar irradiation and of ambient temperature. This paper is an attempt to give a complete design of MPPT considering the various factors like ambient conditions, load etc. The design is done with simulation experimental setup. The PSIM software is used for simulation work. P&O/Hill Climbing MPPT algorithm is used to for the presented work. 40W Solar PV and Buck DC-DC converter is chosen for this design work. The approximate cost is also evaluated. This work may be useful for the design of Solar PV system for Industry and academic application. The paper is organised as following: Section 2 give the details of modified MPPT technque. In Section 3 load analysis with MPPT technique and buck converter is discussed. The experimental and simulation results are discussed in Section 4. DOI: /IJMTER OW9F 38

2 II. MODIFIED PERTURB & OBSERVE MPPT TECHNIQUE There is a minor difference between P&O and Hill climbing method. In P&O the perturbation is given either in the PV voltage or in the PV current ( either of which is taken as a reference signal), but in hill climbing the perturbation is given in the duty cycle which indirectly perturbs the reference signal [5-6]. The flow chart of the P&O/ Hill Climbing method is shown in Fig (1). The main aim is to reach to the MPP. To achieve it the system operating point is changed by applying a small perturbation in the duty cycle. After each perturbation the power output is measured. If the value of power measured is more than the previous value then the perturbation is continued in the same direction. At any point if the new value of solar PV power is measured less than the previous one then perturbation is to apply in the opposite direction. This process is continued till MPP is reached. The issue with this method is it becomes oscillatory around MPP. These oscillations can be minimized by reducing the step size of perturbation. But care should be taken that a smaller step may slow down the MPPT. Start Measure V cell, I cell Record Power (P) Perturb duty cycle (Till it reaches 96%) Maximum Power Point as operating point P t Perturb duty cycle Measure Power P t+1 No Compare P (t+1) P (t) >0 Perturb duty cycle in the reverse direction Yes Make P t+1 as the new operating point Perturb duty cycle in the same direction Fig. 1: Flowchart of Modified Perturb & Observe/Hill Climbing MPPT method. III. LOAD ANALYSIS OF MPPT WITH BUCK DC-DC CONVERTER MPPT algorithm is implemented to harvest the maximum power from sun through solar PV under an ambient condition. It is essential for researcher and design engineer to evaluate MPPT with load variations also. In the previous section it is elaborated that by varying the duty cycle of dc dc converter (Buck converter used in this paper) the impedance is matched to transfer the maximum power from solar PV to load. For dc dc buck converter the maximum value of duty cycle is 1 and if R Load increases beyond a certain value then MPP will not work. This certain value of R Load is R MPP (V MPP /I MPP ). If the R Load >> R MPP the duty cycle variation will fail to match it will R in. This may lead to the failure of MPP. It can be understood in other terms i.e. of the load power is less than the MPP power then MPP fails, meaning which solar PV is derated. Table I gives the summary of MPP limitation. Fig. 2 shows the two zones i.e. MPP ZONE and NO MPP ZONE on the V I Characteristic of solar All rights Reserved 39

3 Fig. 2: MPP Zones of Solar PV (I V) Curve. The Buck converter works in the following two modes: A. When MPPT is working During this mode of operation the input voltage is V MPP and the output voltage varies depending on duty cycle. As described earlier the range of duty cycle is from 4% to 92%. i.e. the output voltage varies between 0.4V MPP to 0.92V MPP. B. In No MPPT Zone During this mode of operation the input voltage is either greater than V MPP or less than or equal to open circuit voltage Voc. So the output voltage in this mode varies with duty cycle times the input voltage. These two operating conditions are summarized in Table II. While designing the buck dc-dc converter the important factor is to calculate the value of inductor keeping the frequency of operation and duty cycle range in mind. The other aim is to calculate the maximum inductor current to decide the saturation limits and also the allowable ripples in the output voltage [10]. The parameters of buck dc-dc converter are given in Table II. S.No. R Load MPP Scheme P Load 1 < R MPP Working = P MPP 2 = R MPP Working = P MPP 3 > R MPP Fail < P MPP TABLE I: PARAMETERS OF DC-DC BUCK CONVERTER S.No. Name of the Parameter Values 1 Vin V MPP : when MPP is working V MPP < Vin Voc : In NO MPP Zone 2 MOSFET 20A, 600V 3 DIODE 12A, 1000V 4 L buck 1mH, 15A Saturation 5 C buck 1000 uf 6 Vo d * V MPP : when MPP is working V MPP < d * Vin Voc : In NO MPP Zone 7 R LOAD Variable 8 Frequency 20 khz 9 Power Output 40W TABLE II: PARAMETERS OF DC-DC BUCK CONVERTER P&O/Hill climbing MPPT scheme is used to control the duty cycle of MOSFET switch (Fig. 2) to regulate the power point at maximum value. Fig. 2: Schematic: DC-DC Buck All rights Reserved 40

4 The role of duty cycle to control the power is analysed below: The maximum power can be transferred with source to load when impedance at source side matches with load impedance. The resistance of solar PV is the input resistance R in for DC-DC converter. Since the MPPT is implemented and the operating point is MPP so the corresponding resistance at input is R MPP. The resistance connected at the output of DC DC converter is R LOAD. R in =R Load /d 2 (1) The relationship between the input resistance, output resistance and duty cycle is shown by equation (1). Its variation is plotted in Fig (3). Fig. 3: Input Resistance versus duty cycle At the maximum power point (MPP) on the PV curve of solar PV the input resistance ( i.e. the resistance of solar PV) is matched with R Load by adjusting the duty cycle with the above empirical relation. Once the R Load is changed the duty cycle is adjusted to make MPP as operating point. The detailed results are given in later section. III. EXPERIMENTAL SETUP The block diagram of experimental setup is shown in Fig. 4. Solar PV is connected with DC- DC buck converter whose output is connected with a load (resistive for this work). The PV terminal voltage and current is sensed using sensors and given as an input to the MPPT controller. The MPPT controller is implemented using microcontroller PIC-16F887. Using this microcontroller PWM signal is generated based on MPPT algorithm and is given to the mosfet of buck converter using and opto-coupler driver IC. The microcontroller is also connected with a data logger to record the parameters. The value of R LOAD is also recorded by measuring the voltage and current at the load end. DC-DC Buck Converter A LOA D + - V PV Opto-Coupler IC: TLP250 Voltmeter I PV Voltage Sensor Current Sensor Microcontroller PWM (PIC-16F887) MPPT Controller Digital CRO DATA LOGGER Fig. 4: Block diagram of Experimental All rights Reserved 41

5 IV. SIMULATION & EXPERIMENTAL RESULTS The P&O/ Hill Climbing MPPT scheme is implemented in the experimental set up of Fig. 4. The P-V and I-V curve of solar PV is plotted shown in Fig. 5 & Fig. 6 respectively. Fig 7 & 8 are simulation and experimental result of PV characteristics. Fig. 5: Simulation Result: Power (P)-Voltage (V) curve. Fig. 06: Simulation Result: Current (I)-Voltage (V) curve. Power Fig. 07: Simulation Result: Solar PV: P-V Curve ( Solar irradiation: 700 W/m 2, Ambient Temp: 35 deg cel) Vcell Fig. 08: Experimental Result: Solar PV: P-V Curve (Solar irradiation: 700 W/m 2, Ambient Temp: 35 deg All rights Reserved 42

6 As described in previous section that duty cycle is perturbed in a step of 4% starting from 8% to 92%. At point D of Fig. 09, 10 the perturbation is started. It keeps perturbing and the operating point is in NO MPP Zone as long as R Load > R MPP. It can be understood that if the load power is less than the MPP power then MPP scheme will not work. Operating zone shown between Point C and D is no MPP operating zone in Fig. 09 and 10. The operating zone left of Point C is the MPP operating zone. The MPP point lies in the vicinity of Point C towards left. The small points in the vicinity of point C are due to the perturbation in the duty cycle and corresponding shift in the operating point. In case I: The load is increased and the operating point is shifted from point C to point A and at this point the MPP algorithm works to adjust the duty cycle and bring the operating point back at MPP point i.e. from A to C. A NO MPP ZONE D Fig. 09: Experimental Result: Solar PV: P-V Curve with MPPT ( Solar irradiation: 700 W/m 2, Ambient Temp: 35 deg cel) NO MPP ZONE D Fig. 10: Experimental Result: Solar PV: P-V Curve with MPPT ( Solar irradiation: 700 W/m 2, Ambient Temp: 35 deg cel) In case II: A small load change gives the same results between point C and point B. The effectiveness of MPPT is checked by changing the load and plotting the curve from the experimental data stored using data logger. The other check of MPP algorithm is under change atmospheric conditions. The experimental result of P-V curve is plotted in Fig. 11. The results are stored under ambient condition of All rights Reserved 43

7 irradiation of 200 W/m 2 and ambient temperature of 22 degree celcius. The result shows that designed MPPT is working effectively. Fig. 11: Experimental Result: Solar PV: P-V Curve with MPPT ( Solar irradiation: 200 W/m 2, Ambient Temp: 22 deg cel) C. Analysis of results: As discussed in previous sections MPPT is designed to adjust the duty cycle for fixing the operating point at MPP when R Load is less than R MPP. Table III gives the experimental results for R Load, R MPP and duty cycle. The variation in duty cycle waveform is captured with change load and is given in Fig. 12, and Fig. 13 respectively. Fig. 12: Experimental Result: Duty Cycle Fig. 13: Experimental Result: Duty cycle when load is All rights Reserved 44

8 The result shown in Table III explains the status of MPP. MPP is working as long as RLOAD/D 2 is equal to R MPP. The point after which R LOAD is greater that R MPP maximum power point racking scheme fails. Fig. 12 is the waveform of the duty cycle. Fig. 13 is the waveform of duty cycle when load is increased. Fig. 14 is the photograph of experimental setup done for this design. TABLE III: MEASURED VALUES OF RLOAD, RMPP AND DUTY CYCLE S.NO. VMPP IMPP RMPP RLOAD DUTY RLOAD/D 2 MPP STATUS (V) (A) (OHM) (OHM) CYCLE (%) WORKING WORKING FAILED Fig. 14: Experimental Setup D. Cost: The major cost of this system is of Solar PV i.e. around $ 150 for 40W panel. The cost of other components used i.e. in DC-DC buck converter, sensors, microcontroller etc is around $ 25. The BOM (bill of material) cost is around $ 175. This cost is a rough BOM cost which does not include the cost of casing, manufacturing etc. E. Safety: In this design the system is operating even at short circuit current because the power is low. In actual practise a safety zone should be defined ( may be 10% from Isc) in which the circuit should be disabled to avoid catastrophic failures. V. CONCLUSIONS In this paper a detail design procedure of solar PV energy conversion system is described. As a sample 40W solar PV is chosen for this work. The experimental data is compared with simulation results and analysed with concepts of MPPT scheme. An approximate cost of the whole design is also discussed. This method may be very useful for industry and researchers to develop and improve the design of solar PV energy conversion system. The same procedure is valid if either the wattage of the system is increased or the power electronics converter is changed. REFERENCES 1. AEO (2013) Annual Energy Report. Department of Energy, USA Solanki S. Chetan. (2012): Solar Photovoltaics: Fundamentals, Technologies and Applications, New Delhi: PHI. 3. M. G. Villalva, J. R. Gazoli, E.R. Filho. (2009). Comprehensive approach to modelling and simulation of photovoltaic arrays. IEEE Transaction on Power Electronics 5: Esram Trishan, Chapman L. Patrick (2007) Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques. IEEE Transaction on Energy Conversion, Vol. 22, No De Brito M A Gomes, Galotto Luigi, Poltronieri Leonardo, E Melo Guilherme de Azevedo e Melo, Canesin carlos Alberto (2013) Evaluation of the Main MPPT Techniques for Photovoltaic Applications. IEEE Transaction on Industrial Electronics, Vol. 60, No. All rights Reserved 45

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